@lof -- Not my exact error, but got me on the right path. Thank you.

Talk about a hard problem to diagnose! I canceled the installation of Rack 1.0 half way through because I realized I was running the wrong command (I didn’t use sudo like I wanted to). After that, I couldn’t load rack at all, even though I could see it in my gems directory and I could load other gems there. I got a LoadError, like this: irb(main):001:0> require 'rubygems' => true irb(main):002:0> require 'rack' LoadError: no such file to load -- rack from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require' from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:31:in `require' from (irb):2 I tried downgrading Rails to the version that used Rack 0.9.1 and then I got an error saying Rails couldn’t activate Rack 0.9.1 because 1.0.0 was already active! Finally figured it out—there was a gemspec for rack-1.0.0 in my ~/.gems directory, but n...